Hydrogen production from fossil fuels and especially by steam reforming of methane or other hydrocarbons is currently the most common process for the production of hydrogen on an industrial scale. In this process, fossil fuel, for example natural gas or methane, is reacted with steam at high temperatures (700-1100° C., typically 700-900° C.) to produce synthesis gas (syngas), a gas mixture primarily made up of hydrogen (H2) and carbon monoxide (CO). Syngas can also be obtained by autothermal reforming or by catalytic partial oxidation of hydrocarbons. Further, the syngas can be reacted with steam at a lower temperature in a water gas shift (WGS) reaction, to form carbon dioxide (CO2) and hydrogen. In this way the hydrogen recovery from the hydrocarbon feed is further increased.
Since in the above reactions CO and CO2 are formed, production of hydrogen is associated with the emission of significant amounts of carbon oxides, which are considered greenhouse gases. In fact, during hydrogen production by natural gas reforming, more CO2 is emitted than H2 produced. In addition, since the reforming reaction employs high temperatures, a considerable amount of fuel is needed to be burnt to maintain the required temperatures, which further contributes to the high CO2 emission.
The carbon dioxide emission and the fuel needed for the combustion can be decreased if the efficiency of the steam reforming process is improved. Currently, the thermal efficiency achieved at existing hydrogen plants is only 65-75% and, therefore, efficiency improvement is desired.
Methods are known in the prior art to improve the efficiency of the steam reforming processes. For example, US2008/0000350 A1 describes a method for hydrogen production wherein the water gas shift reaction is performed in an integrated water gas shift/hydrogen separation membrane system. In this system, hydrogen is separated in the same reactor where the water gas shift reaction occurs, which improves the process efficiency.
G. Barigozzi et al., Int Journal of Hydrogen energy, 36 (2011), 5311-5320 discloses several configurations wherein a membrane separation unit is placed before and after the water gas shift reactor or before the PSA unit. Barigozzi concludes that a configuration with the membrane unit placed after water gas shift reactor leads to the most efficient overall process.
Although attempts have been made to increase hydrogen production and efficiency in steam reforming, it is still desirable to further improve the overall efficiency of the process and the hydrogen recovery from a hydrocarbons-containing feed. In addition, it is desired to produce hydrogen having a lower caloric value of the feed and fuel needed to produce a volume of hydrogen (kcal/Nm3). Hydrogen produced with a low caloric value of the feed is associated with lower production costs and a lower emission of carbon oxides caused by the production.